1. Field of Invention
This invention pertains to modems used in data communication, and more particularly to a QAM or PSK modem which is able to generate signals indicating the received signals and line or channel impairments as a composite image.
2. Description of the Prior Art
It has been previously known that the performance of QAM or PSK data transmission systems, and especially the data transmission channels may be evaluated qualitatively by converting the received signals into a corresponding signal constellation and displaying the signal constellation as a two-dimensional pattern. The pattern can exhibit certain characteristics or distortions which are unique to particular line impairments. Thus, for example, a continuous (oscillatory) angular displacement of all the constellation points is indicative of phase jitter; continuous (oscillatory) radial displacement of the points is indicative of harmonic distortion; a relatively constant angular displacement of the constellation points is indicative of frequency offset, and so on. A complete description of characteristic variations in a signal constellation is found in commonly assigned U.S. Pat. No. 4,503,545, granted on Mar. 5, 1985 and entitled "Systems for Evaluating Transmission Line Impairments". In addition, while visual inspection of the constellation is sufficient for QAM or PSK schemes using a relatively small number of points (i.e. 8-16 points) it has been found that for large signal constellations having 64 or more points, visual inspection becomes so difficult that it is virtually impossible to extract any useful information therefrom.